How can I stack all faces of an object on top of each other in the UV Image Editor?
In Edit mode select the subdivided plane:



Open the UV Mapping menu and choose the Reset option.
Each quad of the subdivided plane will now be    unwrapped and mapped to the complete UV space. (Each quad will be stacked one above the other).


After resetting the Uv's sometime one ot more of the islands will be incorrectly orientated to fix these :

In the 3D editor, Face select mode, select a face that needs to be corrected. In the UV editor the appropriate island will be displayed, Select this face with the A key then rotate plus or minus 90 or 180°.